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<h1 id="firstHeading" class="firstHeading">Node handle <span class="t-mark-rev t-since-cxx17">(C++17)</span></h1>
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<table class="t-dcl-begin"><tbody>
<tr class="t-dcl t-since-cxx17">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="coMULTI">/*unspecified*/</span><span class="sy1">&gt;</span><br>
<span class="kw1">class</span> <span class="coMULTI">/*node-handle*/</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span> </td>
</tr> 
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Associative containers <span class="t-lc"><a href="set.html" title="cpp/container/set">std::set</a></span>, <span class="t-lc"><a href="map.html" title="cpp/container/map">std::map</a></span>, <span class="t-lc"><a href="multiset.html" title="cpp/container/multiset">std::multiset</a></span>, <span class="t-lc"><a href="multimap.html" title="cpp/container/multimap">std::multimap</a></span>, <span class="t-lc"><a href="unordered_set.html" title="cpp/container/unordered set">std::unordered_set</a></span>, <span class="t-lc"><a href="unordered_map.html" title="cpp/container/unordered map">std::unordered_map</a></span>, <span class="t-lc"><a href="unordered_multiset.html" title="cpp/container/unordered multiset">std::unordered_multiset</a></span>, <span class="t-lc"><a href="unordered_multimap.html" title="cpp/container/unordered multimap">std::unordered_multimap</a></span> are node-based data structures, and their nodes can be extracted as an object of unspecified type known as <i>node handle</i>.
</p><p>Node handle is a move-only type that owns and provides access to the element (the <code>value_type</code>) stored in the node, and provides non-const access to the key part of the element (the <code>key_type</code>) and the mapped part of the element (the <code>mapped_type</code>). If the node handle destructs while holding the node, the node is properly destructed using the appropriate allocator for the container. The node handle contains a copy of the container’s allocator. This is necessary so that the node handle can outlive the container.
</p><p>The exact type of node handle (shown here as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*node-handle*/</span></span></span>) is unspecified, but each container exposes its node handle type as the member <code>node_type</code>.
</p><p>Node handles can be used to transfer ownership of an element between two associative containers with the same key, value, and allocator type (ignoring comparison or hash/equality), without invoking any copy/move operations on the container element (this kind of operation is known as "splicing"). Transfer between unique and non-unique containers is also permitted: a node handle from a <span class="t-lc"><a href="map.html" title="cpp/container/map">std::map</a></span> can be inserted into an <span class="t-lc"><a href="multimap.html" title="cpp/container/multimap">std::multimap</a></span>, but not into <span class="t-lc"><a href="unordered_map.html" title="cpp/container/unordered map">std::unordered_map</a></span> or <span class="t-lc"><a href="set.html" title="cpp/container/set">std::set</a></span>.
</p><p>A node handle may be empty, in which case it holds no element and no allocator. The default-constructed and moved-from node handle is empty. In addition, an empty node handle can be produced by a failed call to container member function <code>extract</code>.
</p><p>Pointers and references to an element that are obtained while it is owned by a node handle are invalidated if the element is successfully inserted into a container.
</p><p>For all map containers (<span class="t-lc"><a href="map.html" title="cpp/container/map">std::map</a></span>, <span class="t-lc"><a href="multimap.html" title="cpp/container/multimap">std::multimap</a></span>, <span class="t-lc"><a href="unordered_map.html" title="cpp/container/unordered map">std::unordered_map</a></span>, and <span class="t-lc"><a href="unordered_multimap.html" title="cpp/container/unordered multimap">std::unordered_multimap</a></span>) whose <code>key_type</code> is <code>K</code> and <code>mapped_type</code> is <code>T</code>, the behavior of operations involving node handles is undefined if a user-defined specialization of <span class="t-lc"><a href="../utility/pair.html" title="cpp/utility/pair">std::pair</a></span> exists for <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/pair.html"><span class="kw1099">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span>K, T<span class="sy1">&gt;</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../utility/pair.html"><span class="kw1099">std::<span class="me2">pair</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> K, T<span class="sy1">&gt;</span></span></span>.
</p>
<h3><span class="mw-headline" id="Member_types">Member types</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc-hitem">
<td>  Member type
</td>
<td>  Definition
</td></tr>


<tr class="t-dsc">
<td>  <code>key_type</code><span class="t-mark">(map containers only)</span>
</td>
<td>  the key stored in the node </td></tr>

<tr class="t-dsc">
<td>  <code>mapped_type</code><span class="t-mark">(map containers only)</span>
</td>
<td>  the mapped part of the element stored in the node </td></tr>

<tr class="t-dsc">
<td>  <code>value_type</code><span class="t-mark">(set containers only)</span>
</td>
<td>  the element stored in the node </td></tr>

<tr class="t-dsc">
<td>  <code>allocator_type</code>
</td>
<td>  the allocator to be used when destroying the element </td></tr>
</table>
<h3><span class="mw-headline" id="Member_functions">Member functions</span></h3>
<div class="t-member">
<h2> <span class="mw-headline" id="constructors"> constructors </span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="coMULTI">/*node-handle*/</span><span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td> (1) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*node-handle*/</span><span class="br0">(</span><span class="coMULTI">/*node-handle*/</span><span class="sy3">&amp;&amp;</span> nh<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td> (2) </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<div class="t-li1"><span class="t-li">1)</span> The default constructor initializes the node handle to the empty state.</div>
<div class="t-li1"><span class="t-li">2)</span> The move constructor takes ownership of the container element from <code>nh</code>, move-constructs the member allocator, and leaves <code>nh</code> in the empty state.</div>
<h3> <span class="mw-headline" id="Parameters">Parameters</span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  nh
</td>
<td> -
</td>
<td>  a node handle with the same type (not necessarily the same container)
</td></tr></table>
<h3> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Node handles are move-only, the copy constructor is not defined.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="operator.3D"> operator=</span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*node-handle*/</span><span class="sy3">&amp;</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="coMULTI">/*node-handle*/</span><span class="sy3">&amp;&amp;</span> nh<span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<ul><li> If the node handle is not empty, 
</li></ul>
<dl><dd><ul><li> destroys the <code>value_type</code> subobject in the container element object managed by this node handle by calling <code>std::allocator_traits&lt;allocator_type&gt;::destroy</code>;
</li><li> deallocates the container element by calling <code>allocator_traits&lt;allocator_type&gt;::rebind_traits&lt;container-node-type&gt;::deallocate</code>;
</li></ul>
</dd></dl>
<ul><li> Acquires ownership of the container element from <code>nh</code>;
</li><li> If node handle was empty (and so did not contain an allocator) or if <code>allocator_traits&lt;allocator_type&gt;::propagate_on_container_move_assignment</code> is <code>true</code>, move-assigns the allocator from <code>nh</code>;
</li><li> sets <code>nh</code> to the empty state.
</li></ul>
<p>The behavior is undefined if the node is not empty and <code>allocator_traits&lt;allocator_type&gt;::propagate_on_container_move_assignment</code> is <code>false</code> and the allocators do not compare equal.
</p>
<h3> <span class="mw-headline" id="Parameters_2">Parameters</span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  nh
</td>
<td> -
</td>
<td>  a node handle with the same type (not necessarily the same container)
</td></tr></table>
<h3> <span class="mw-headline" id="Return">Return</span></h3>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span>
</p>
<h3> <span class="mw-headline" id="Exceptions">Exceptions</span></h3>
<p>Throws nothing.
</p>
<h3> <span class="mw-headline" id="Notes_2">Notes</span></h3>
<p>Node handles are move-only, the copy assignment is not defined.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="destructor"> destructor </span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp">~<span class="coMULTI">/*node-handle*/</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<ul><li> If the node handle is not empty, 
</li></ul>
<dl><dd><ul><li> destroys the <code>value_type</code> subobject in the container element object managed by this node handle by calling <code>std::allocator_traits&lt;allocator_type&gt;::destroy</code>;
</li><li> deallocates the container element by calling <code>allocator_traits&lt;allocator_type&gt;::rebind_traits&lt;container-node-type&gt;::deallocate</code>.
</li></ul>
</dd></dl>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="empty"> empty</span></h2>
<table class="t-dcl-begin"><tbody>
</tbody>
<tbody class="t-dcl-rev">
<tr class="t-dcl t-until-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> empty<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-until-cxx20">(until C++20)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">bool</span> empty<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx20">(since C++20)</span> </td>
</tr>
</tbody><tbody>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Returns <code>true</code> if the node handle is empty, <code>false</code> otherwise.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="operator_bool"> operator bool</span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Converts to <code>false</code> if the node handle is empty, <code>true</code> otherwise.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="get_allocator"> get_allocator</span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp">allocator_type get_allocator<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Returns a copy of the stored allocator (which is a copy of the allocator of the source container). The behavior is undefined if the node handle is empty.
</p>
<h3> <span class="mw-headline" id="Exceptions_2">Exceptions</span></h3>
<p>Throws nothing.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="value"> value </span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp">value_type<span class="sy3">&amp;</span> value<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark">(set containers only)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Returns a reference to the <code>value_type</code> subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.
</p>
<h3> <span class="mw-headline" id="Exceptions_3">Exceptions</span></h3>
<p>Throws nothing.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="key"> key </span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp">key_type<span class="sy3">&amp;</span> key<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark">(map containers only)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Returns a non-const reference to the <code>key_type</code> member of the <code>value_type</code> subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.
</p>
<h3> <span class="mw-headline" id="Exceptions_4">Exceptions</span></h3>
<p>Throws nothing.
</p>
<h3> <span class="mw-headline" id="Notes_3">Notes</span></h3>
<p>This function makes it possible to modify the key of a node extracted from a map, and then re-insert it into the map, without ever copying or moving the element.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="mapped"> mapped </span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td> <div><span class="mw-geshi cpp source-cpp">mapped_type<span class="sy3">&amp;</span> mapped<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark">(map containers only)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Returns a reference to the <code>mapped_type</code> member of the <code>value_type</code> subobject in the container element object managed by this node handle. The behavior is undefined if the node handle is empty.
</p>
<h3> <span class="mw-headline" id="Exceptions_5">Exceptions</span></h3>
<p>Throws nothing.
</p>
</div>
<div class="t-member">
<h2> <span class="mw-headline" id="swap"> swap </span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> swap<span class="br0">(</span><span class="coMULTI">/*node-handle*/</span><span class="sy3">&amp;</span> nh<span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<ul><li> swaps ownership of container nodes;
</li><li> if one node is empty or if both nodes are non-empty and <code>std::allocator_traits&lt;allocator_type&gt;::propagate_on_container_swap</code> is <code>true</code>, swaps the allocators as well.
</li></ul>
<p>The behavior is undefined if both nodes are not empty and <code>allocator_traits&lt;allocator_type&gt;::propagate_on_container_swap</code> is <code>false</code> and the allocators do not compare equal.
</p>
<h3> <span class="mw-headline" id="Exceptions_6">Exceptions</span></h3>
<div class="t-noexcept-full"><a href="../language/noexcept_spec.html" title="cpp/language/noexcept spec"><tt>noexcept</tt></a> specification:  
<div class="t-noexcept-box"><span class="t-cc"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span><a href="../memory/allocator_traits.html"><span class="kw702">std::<span class="me2">allocator_traits</span></span></a><span class="sy1">&lt;</span>allocator_type<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">propagate_on_container_swap</span><span class="sy4">::</span><span class="me2">value</span> <span class="sy3">||</span><br>
 <a href="../memory/allocator_traits.html"><span class="kw702">std::<span class="me2">allocator_traits</span></span></a><span class="sy1">&lt;</span>allocator_type<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">is_always_equal</span><span class="sy4">::</span><span class="me2">value</span><span class="br0">)</span></span></span></div></div>
</div>
<h3><span class="mw-headline" id="Non-member_functions">Non-member functions</span></h3>
<div class="t-member">
<h2> <span class="mw-headline" id="swap_2"> swap </span></h2>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">friend</span> <span class="kw4">void</span> swap<span class="br0">(</span><span class="coMULTI">/*node-handle*/</span><span class="sy3">&amp;</span> x, <span class="coMULTI">/*node-handle*/</span><span class="sy3">&amp;</span> y<span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="kw1">noexcept</span><span class="br0">(</span>x.<span class="me1">swap</span><span class="br0">(</span>y<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td class="t-dcl-nopad">  </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Effectively executes <span class="t-c"><span class="mw-geshi cpp source-cpp">x.<span class="me1">swap</span><span class="br0">(</span>y<span class="br0">)</span></span></span>.
</p><p>This function  is not visible to ordinary <a href="../language/unqualified_lookup.html" title="cpp/language/unqualified lookup">unqualified</a> or <a href="../language/qualified_lookup.html" title="cpp/language/qualified lookup">qualified lookup</a>, and can only be found by <a href="../language/adl.html" title="cpp/language/adl">argument-dependent lookup</a> when <code><i>node-handle</i></code> is an associated class of the arguments.
</p>
</div>

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